When liquid is stored in a tank, the pressure in a gas-phase portion of the tank is variable, depending on the temperature of the stored liquid, on the reaction in a reaction portion of a battery cell, and the like. When the pressure difference between the inside and the outside of the tank is strictly limited, as in the case of a polyethylene tank etc., it is possible to make the tank a sealed structure by providing an air release valve or the like, for positive pressure. Furthermore, if the internal pressure in the tank may become negative because of a temperature drop of the stored liquid, the negative pressure is generally avoided by communicating with outside air or providing an intake valve to take in outside air.
When the stored liquid in the tank cannot be exposed to the outside air, for example when it is extremely vulnerable to oxidation, however, it is necessary to blow in and flow inert gas such as nitrogen so as to prevent the negative pressure. As a result, the inert gas for preventing the negative pressure must be reserved, resulting in an undesired cost increase.
If a sealed structure filled with inert gas is employed to prevent this, degradation including oxidization of the stored liquid by air can be avoided, while preventing the negative pressure. When the stored liquid is electrolyte for a redox flow secondary battery or the like, an endothermic reaction at the time of charging and an exothermic reaction at the time of discharging, as well as changes of outside air temperature raises the temperature of the stored liquid up to about 42° C. in operation and lowers down to about 10° C. at rest. Therefore, as temperature varies in the range of 10° C. to 42° C., pressure accordingly varies in the sealed structure. When we represent the variation of pressure P as ΔP and the variation of temperature T (absolute temperature) as ΔT, (ΔP/P)=(ΔT/T) holds, since volume change ΔV is zero in a sealed structure. By inputting the above value, (ΔP/P)={32/(273+42)}≈0.1 results, and the above noted temperature variation causes a variation of about 0.1 atmosphere in operation under normal pressure. If an electrolyte tank for a redox flow secondary battery, for example, is to withstand this pressure variation of about 0.1 atmosphere, the structure thereof needs to be considerably rigid, which boosts up the cost of the entire battery system. The pressure-resistant performance of an inexpensive polyethylene tank or the like, fabricated by rotational molding etc. is at most about 0.01 atmosphere for positive pressure variation, and no more than about 0.005 atmosphere for negative pressure variation. In case of the electrolyte tank for the redox flow secondary battery, it is impossible to implement a tank having so rigid a structure that can withstand the pressure variation as mentioned above, due to the constraint of locational conditions or power generation costs.